Trade Offs and Benefits of Direct versus Indirect Air Economization in Data Centres

Using outdoor air to assist in the cooling of data centres can lower energy costs and carbon footprint but care must be taken to gain maximum benefit. White Paper 215 from Schneider Electric discusses the difference between direct and indirect economizer modes and the optimum situations for each method.

Cooling equipment such as chillers and compressors are important components of the process of cooling IT equipment in a data centre. However they are heavy users of both energy and refrigerant materials, which push up costs and increase carbon footprint.

Using outdoor air in the cooling process allows a data centre to run in economizer mode, in which the load on chillers and condensers can be greatly reduced or even removed. The choice between using direct or indirect economization depends on a number of factors particular to each data centre, including the ambient climactic conditions, the degree of risk associated with availability and costs.

Co-written by Paul Lin, John Niemann and Leo Long of Schneider Electric, White Paper #215 “Choosing Between Direct and Indirect Air Economization for Data Centers” describes in detail the differences between each approach and recommends scenarios in which each is the preferable option.

Direct air economization uses fans and louvers to draw cold external air through filters and then directly into the data centre when outside air conditions are within specified set points. Such systems use dampers to control the amount of hot air that is exhausted to the outdoors and mixed back into the air supplying the data centre so that the environmental set points are maintained.

Indirect air economization also draws in cold external air when outside conditions are within specified set points but in this case air-to-air heat exchangers, heat wheels or heat pumps are used to isolate the impact of outside humidity and filter pollutants before the air enters the IT space. There, fans blow the cold air over a series of plates or tubes through which hot data centre air is running and help to cool it.

White Paper 215 covers in depth the pros and cons of each mode. Direct air economization has lower capital costs for prefabricated systems because it does not use an air-air heat exchange stage to isolate external air from the air in the IT equipment space.

Direct air economization can be a cost-effective option for geographical regions with cool temperatures, low humidity and air quality with low levels of pollutants. However, such systems are highly susceptible to changes in outdoor air quality and costs can increase if extra filters are needed or if input air has to be treated for humidity.

Indirect air economization has higher capital costs but typically these are only a small percentage of the overall cost of a data centre. Operating and maintenance costs are lower as the outdoor air is completely isolated from the data centre air by the heat exchangers. The heat-exchange phase also lowers the efficiency of indirect air economization compared with direct options but availability risks are also lower.